Solderable top metalization and passivation for source mounted package

ABSTRACT

A silver-containing solderable contact on a semiconductor die has its outer edge spaced from the confronting edge of an epoxy passivation layer so that, after soldering, silver ions are not present and are not therefor free to migrate under the epoxy layer to form dendrites.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/552,139, filed Mar. 11, 2004.

FIELD OF THE INVENTION

This invention relates to semiconductor devices and processes for their manufacture and more specifically relates to a novel solderable contact structure for semiconductor die.

BACKGROUND OF THE INVENTION

Solderable contacts for semiconductor devices are well known. Such contact structures are commonly silver-containing alloys deposited on aluminum die electrodes, which are insulated from other surfaces by an insulation passivation coating which overlaps the edges of the contact area.

It has been found that the silver ions from the top metal layer will migrate under the passivation layer and form dendrites under prolonged exposure to electric fields and moisture. Thus, over time, the dendrites will form conductive bridges between device electrodes and device terminations, thus reducing device reliability.

It would be desireable to provide a passivated top silver-containing solderable contact in which the migration of silver from device electrodes and under the passivation is prevented.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the invention the silver-containing solderable metal layer is terminated around its periphery and is spaced from the edge an epoxy passivation layer, forming a gap between the edge of the epoxy passivation and the confronting edge of the solderable metal layer. During solder attach of the device to a circuit board or the like, the attach-solder will dissolve the exposed silver, forming a solder alloy. This prevents the migration of silver ions from the solderable electrodes to the termination during applied electric fields to the device during its operation. Thus, dendrite formation is reduced and device reliability is improved.

The novel invention has application to any semiconductor device in which a solderable contact is desired, such as the Direct FET® device of International Rectifier as shown, for example, in U.S. Pat. No. 6,624,522, issued Sep. 23, 2003, entitled CHIP SCALE SURFACE MOUNTED DEVICE AND PROCESS OF MANUFACTURE (IR-1830), as well as to flip chips; bumped/wafer level packages and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bottom view of a Direct FET® type package of the type shown in aforementioned U.S. Pat. No. 6,624,522.

FIG. 2 is a cross-section of the circled portion of FIG. 1, showing the area of overlap of a passivation layer and solderable metal of the prior art.

FIG. 3 is a cross-section like that of 2, but illustrating the novel invention in which the edge of the solderable metal is spaced from the confronting edge of the epoxy passivation.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to FIGS. 1 and 2 there is shown a bottom view of a Direct FET® type package 10 in which a MOSFET semiconductor die 11 is contained within a metal can 12 which has an open bottom, shown in Figure, and a top web 13, shown in FIGS. 2 and 3. The web 13 is connected to flanges 14 and 15 which form a contact for drain electrode 16 on the bottom surface of die 11.

The top surface of die 11 is exposed by the open bottom of conductive can 12 and contains a source electrode having solderable contacts 20 and 21 and a solderable gate contact 22. The top surfaces of contacts 20, 21 and 22 are generally coplanar with drain contacts 14, 15 although there could be an upset of up to about 50 μm due to tolerance variations. Thus, the device is solderable at its contacts 14, 15, 20, 21 and 22 to respective corresponding contact areas on a flat circuit board (not shown). The contacts 20, 21 and 22 are insulated from one another and from the conventional termination enclosing the upper surface of die 11 by an insulation passivation layer 30.

It is to be noted that the invention is illustrated as applied to a Direct FET® type device. However, the die 11 may be any semiconductor die which has a solderable contact which is insulated by a surrounding passivation coating.

In the device illustrated, the die 10 (shown as silicon, but which may be of other materials, for example, SiC, GaN, and the like) has the contact structure shown in cross-section in FIG. 2, which is a section through the surface area shown in the circle 35 in FIG. 1. Thus solderable contact 21 is the exposed area of a silver-containing solderable metal such as a conventional titanium/nickel/silver stack having thicknesses of 1,000 Å, 2000 Å and 6000 Å respectively, which is deposited atop the aluminum source contact 40 of MOSFET die 11. The solderable contact 21 is surrounded by insulation passivation layer 30 which consists of a nitride layer 42 and an epoxy layer 41. Note that the edge of contact 21 overlaps nitride layer 42 and is encapsulated under the epoxy layer 41. Thus, the outer rim area 43 of contact 21, is captured under epoxy 41 and is not exposed to solder during a soldering operation. The epoxy layer 41 may also be formed of BCB, polyamide or polysiloxane passivation materials.

In the structure of FIG. 2, and after soldering to the exposed area of contact 21, free silver ions can migrate from top metal 21, under the insulation epoxy layer and form dendrites under prolonged exposure to electric fields and moisture during operation. These dendrites can form conductive bridges to and toward the die terminations, thus impacting device reliability.

In accordance with the invention, and as shown in FIG. 3, the edge of metallizing 21 is foreshortened to overlie nitride layer 42, but spaced from the confronting edge of epoxy layer 41, creating the gap 50 there between. The gap 50 will preferably surround the periphery of contact 21 in FIG. 1. Significantly, the outer rim of contact 21 is fully exposed for solder connection.

When the device of FIG. 3 is to be attached by soldering to a circuit board, the attach solder dissolves the exposed silver containing contact 21, forming a solder alloy in the conventional manner. This takes place when using any conventional solder. The silver is now fully captured within the alloy and cannot migrate from the device electrodes to create dendrites and reduce device reliability. Note that in the prior art device that the silver-containing solderable contact 21 captured under the epoxy layer 42 is not reached during soldering and the remaining silver is a source of the migrating ions which will form the disadvantageous dendrites.

To form gap 50, the nitride layer 42 was extended by about 35 microns, as compared to FIG. 2; and the gap 50 is about 10 microns and can be from about 5 microns to about 20 microns wide. More generally, the gap should be at least as wide as needed to insure that no silver is encapsulated under epoxy 41, and all of the silver contact 21 is exposed to alloying during soldering.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein. 

1. A solderable contact for a semiconductor die; said solderable contact comprising a silver-containing layer disposed atop a semiconductor die; and a passivation layer formed on said die surface and generally coplanar with said solderable contact; said passivation layer and said solderable contact having confronting edges which are spaced by a gap.
 2. The solderable contact of claim 1, wherein said passivation layer has an underlying nitride layer; said underling nitride layer extending across and under said gap and being overlapped by said edge of said solderable contact.
 3. The device of claim 1, wherein said passivation layer is an epoxy layer.
 4. The device of claim 2, wherein said passivation layer is an epoxy layer.
 5. The device of claim 1, wherein said solderable layer is a composite layer of titanium, nickel and silver.
 6. The device of claim 2, wherein said solderable layer is a composite layer of titanium, nickel and silver.
 7. The device of claim 3, wherein said solderable layer is a composite layer of titanium, nickel and silver.
 8. The device of claim 4, wherein said solderable layer is a composite layer of titanium, nickel and silver.
 9. The device of claim 1, wherein said gap has a width of from about 5 microns to about 20 microns.
 10. The device of claim 2, wherein said gap has a width of from about 5 microns to about 20 microns.
 11. The device of claim 3, wherein said gap has a width of from about 5 microns to about 20 microns.
 12. The device of claim 5, wherein said gap has a width of from about 5 microns to about 20 microns.
 13. A semiconductor device comprising a silicon die having a top surface, a termination region, and a plurality of solderable contacts disposed on said top surface and spaced from said termination region; an insulation passivation layer on said top surface and surrounding and coplanar with said solderable contacts; the surfaces of said solderable contacts being exposed for solder connection; and a gap disposed between the confronting edges of said passivation layer and each of said solderable contacts.
 14. The device of claim 13, wherein said solderable contacts are silver-containing contacts; and whereby, after soldering, the exposed edges of said solderable contacts are fully converted to a solder alloy which prevents the production of silver ions which can travel under said passivation layer to said termination region during device operation.
 15. The device of claim 13, which further includes a nitride layer which underlies said passivation layer and the edges of said solderable contacts.
 16. The device of claim 15, wherein said gaps are from about 5 microns to about 20 microns long.
 17. The device of claim 15, wherein said solderable contacts are stacked TiNiAg contacts.
 18. The device of claim 16, wherein said solderable contacts are stacked TiNiAg contacts.
 19. The process of preventing the formation of silver dendrites due to the migration of silver ions from a silver-containing solderable electrode atop a semiconductor die to a passivation layer on said die during device operation; said process comprising the formation of a gap between the edge of said electrode and the confronting edge of said passivation layer whereby, during soldering of said solderable electrode to a support, substantially all the silver of said contact is converted to a solder alloy to reduce the possibility of migration of silver ions beneath said passivation layer.
 20. The process of claim 19, wherein said gap has a length of from 5 microns to about 20 microns.
 21. The device of claim 1, wherein said generally coplanar passivation layer and solderable contact are offset by up to about 50 μm to fully expose the surface of said solderable contact.
 22. The device of claim 1, wherein said passivation layer is of a material selected from the group consisting of epoxies, BCB, polymides and polysiloxane.
 23. The device of claim 21, wherein said passivation layer is of a material selected from the group consisting of epoxies, BCB, polymides and polysiloxane. 